The process coils and the fire tubes are installed inside of a pressure vessel with two distinctive sections. The fire tube is immersed in water in the lower section of the vessel, while the process coil is located in the upper section of the vessel above the water level. During operation, heat is transferred to the water in the lower zone and causes the water to vaporize. This creates a steam zone in the upper section of the vessel which surrounds the process coils. This steam surrounding the process coil will condense on the process coil and fall back into the liquid zone below. This condensing phase change creates an exponential and uniform transfer of heat to the process without any potential for exceeding the temperature limitations of the process fluid. This indirect method of heating, combined with the high heat transfer rate, provides safe, uniform, and regulated heating of the process fluid.